1. Field of the Invention
The present invention relates to an image processing system for performing a predetermined image processing on a multi-tone-level image.
2. Description of Related Art
Recently, as a method for converting a gray image into a binary image, methods disclosed in Japanese Laid-Open Patent Application Nos.5-189610 and 5-197838 have been proposed. In the method of Japanese Laid-Open Patent No.5-189610, the entire image is divided into small regions, and a threshold value is obtained for each region from a tone distribution of adjacent regions. An image is converted into a binary image for each region using the respective threshold value. Thus, it is intended to convert a gray image into a binary image.
In the method of Japanese Laid-Open Patent Application No.5-197838, a dynamic threshold value is calculated from image information of a small reference region, for example, a 3-by-3 pixel local region or a 5-by-5 pixel local region. The dynamic threshold value is used for converting image information of a character image into image information of a binary image. Thus, it is intended that, in the process of converting a character region image into a binary image, a high image resolution for details of characters and line charts/diagrams is maintained.
In some cases, a specific image is present in a background pattern in such a way that it is not easy to clearly recognize or detect the specific image. For example, in FIG. 7, a circle (specific image) is present in parallel oblique lines (background pattern). In such a case, the specific image is partially hidden by the background pattern. In such a case, it is required that the influence of the background pattern is removed and a binary image of the specific image is obtained, as shown in FIG. 11.
However, in the method of Japanese Laid-Open Patent Application No.5-189610, when the background pattern is very fine, each small region divided as mentioned above includes the pattern and the threshold value for each small region is determined by the pattern. Thereby, the threshold value of each region may not be useful for converting an image of the background pattern into an appropriate binary image. For example, it is not possible to remove the background pattern using such threshold values. As a result, it is not possible to extract only a specific image (to be detected) as being a binary image as shown in FIG. 11.
In the method of Japanese Laid-Open Patent Application No.5-197838, the size of a mask such as a 3-by-3 pixel region or a 5-by-5 pixel region is so small that, when a background pattern is present, influence thereof is very large and a suitable threshold value for converting an image into a binary image cannot be obtained.
Thus, in the proposed image processing methods, in a case where the background pattern is fine or the like, it is not possible to obtain an appropriate binary image. In the binary image, a specific image (to be detected) may be broken at the points of the fine pattern, as shown in FIG. 8.